MISFIRES ON 3 CYLINDERS

I have a 2000 Pontiac Grand Am Se with 172,000 miles. It has a misfire on 3 cylinders. At idle it runs rough and low. As I accelerate more it seems to be much smoother. Also I get horrible gas mileage when it's supposed to get 20-32 mpg. I've replaced the plugs, wires, coils, and the ignition module. It also shows no signs of a leaking or blown head gasket. Could it be the fuel injectors or computer?